The invention relates to a method of generating white light pulses (WL pulses) with a white light generation device (WLG device). In particular, the invention relates to a method of filament-based white light generation using a non-linear optical frequency conversion of pump laser pulses in a white light generation crystal (WLG crystal). Furthermore, the invention relates to a WLG device, in particular including a WLG crystal for filament-based generation of white light pulses. Applications of the invention are available e.g. in the fields of generating seed light pulses for optical parametric chirped-pulse amplification (OPCPA) of laser pulses, physical measurement techniques, like white light interferometry or spectrometry, metrology and material processing using light pulses.
For describing the background of the invention, particular reference is made to the following publications:    [1] US 2010/0321767 A1;    [2] US 2003/0147122 A1;    [3] US 2008/0225383 A1;    [4] M. Emons et al. in “Optics Express” vol. 18, 2010, p. 1191-1196;    [5] R. Riedel et al. in “Optics Express” vol. 21, 2013, p. 28987-28998;    [6] R. Riedel et al. in “Optics Letters” vol. 39, 2014, p. 1422-1424; and    [7] M. Bradler et al. in “Appl. Phys. B” vol. 97, 2009, p. 561-574.
It is generally known that ultrashort laser pulses (laser pulses having a pulse duration below 100 fs) have a broad range of applications, which make use of the short pulse durations associated with a broadband spectrum and a high peak-power of the individual pulses. Depending on the particular application, there is an interest in creating the pulses with maximum amplitude, minimum pulse duration and/or minimum pulse to pulse amplitude fluctuations.
Typically, high-power ultrashort laser pulses are created using OPCPA-sources, wherein a broadband seed pulse is amplified with a narrowband pump laser pulse in a non-linear optical crystal. Using a non-collinear geometry of the seed pulses and pump laser pulses (non-collinear OPA, NOPA, see [1] to [6]), a broad range of wavelengths can be amplified with the pump laser pulses, thus allowing the creation of ultrashort laser pulses with a duration below 10 fs. The pump laser pulses are created e.g. with a solid state laser oscillator, like a Ti-Sapphire oscillator or a fiber oscillator, in combination with a fiber amplifier and/or a regenerative pulse amplifier and/or a multipass amplifier. The broadband seed pulses can be generated by a white light generation process (WLG process, see e.g. [7]), which can be driven with a fraction of the pump laser pulses. The WLG process includes focusing laser pulses having a duration in a range of e.g. 100 fs to 1 ps into a non-linear optical crystal, where a so-called light filament is created delivering pulses with a spectral bandwidth from the visible to the infrared region (supercontinuum pulses, WL pulses).
In practice, an instability of the WLG process has been found, including variations or interruptions of the WLG process and resulting in limited applications of the NOPA technique. A certain improvement of long-term stability has been described in [4]. However, this long-term stability is restricted to a range of hours, and it can be obtained with a sophisticated optical set-up operated by an experienced user only. On the contrary, routine applications of ultrashort laser pulses would require a continuous operation on a timescale of days or more without user adjustments even with changing surrounding conditions. Long-term stabilization of an optical parametric chirped-pulse amplifier also has been described in [5]. However, the stabilization was related to the OPCPA process as such rather than to the WLG process.
Stabilized WLG processes are required not only in the generation of high-power laser pulses, but also with other applications of ultrashort laser pulses, e.g. in metrology or for physical measuring techniques.